English: A look at how the octanol/water coefficient (log Kow) of an organic compound affects it's ability to diffuse across the aqueous boundary layer and the gill epithelial cells of a fish. Barriers to diffusion of organic molecules in the gill based on log KOW of the xenobiotic. Green arrows show diffusion across the barriers where the aqueous diffusion boundary will slow the uptake of highly lipophilic molecules (log KOW greater than ~3) and the lipid bilayer of the cell membrane will slow the uptake of highly polar molecules (log KOW less than ~1).